Let’s Encrypt からのメール．
Hello,
Action may be required to prevent your Let’s Encrypt certificate renewals from breaking.
If you already received a similar e-mail, this one contains updated information.
Your Let’s Encrypt client used ACME TLS-SNI-01 domain validation to issue a certificate in the past 60 days. Below is a list of names and IP addresses validated (max of one per account):
mish.myds.me (27.92.169.109) on 2018-12-30
TLS-SNI-01 validation is reaching end-of-life. It will stop working temporarily on February 13th, 2019, and permanently on March 13th, 2019.
Any certificates issued before then will continue to work for 90 days after their issuance date.
You need to update your ACME client to use an alternative validation method (HTTP-01, DNS-01 or TLS-ALPN-01) before this date or your certificate renewals will break and existing certificates will start to expire.
Our staging environment already has TLS-SNI-01 disabled, so if you’d like to test whether your system will work after February 13, you can run against staging: https://letsencrypt.org/docs/staging-environment/
If you’re a Certbot user, you can find more information here:
https://community.letsencrypt.org/t/how-to-stop-using-tls-sni-01-with-certbot/83210
Our forum has many threads on this topic. Please search to see if your question has been answered, then open a new thread if it has not:
https://community.letsencrypt.org/
For more information about the TLS-SNI-01 end-of-life please see our API
announcement:
https://community.letsencrypt.org/t/february-13-2019-end-of-life-for-all-tls-sni-01-validation-support/74209
Thank you,
Let’s Encrypt Staff

どうも TLS-SNI-01 が利用できなくなると言うことらしい．
それで Synology NAS と Let’s Encrypt で検索すると
Synology TLS-SNI-01 End of Life Email? – Help – Let’s Encrypt …
今（2019/01/27）の時点では，はっきりした対応方法が読み取れなかった．
取りあえず，「証明書の更新」を行って 3 ヶ月後に延ばした．

2019/04/03　自動的に更新されたことを確認．